The present procedure for recording shipboard gravity data has been to use a single shipboard gravity meter. The shipboard gravity meter is essentially the same as a land gravity meter, except provisions are made to keep the meter level while in a marine mode and corrections must be made for the movement of the boat. Eotvos Corrections must be calculated from the navigation data to account for changes in the gravity readings caused by course and velocity changes of the recording boat.
The object of our invention is to improve the recording of the shipboard gravity data by using two gravity meters. Both meters are placed near the center of gravity or mass in the recording vessel with one meter faced forward, which is the customary direction, and one meter faced toward the stern (the back of the boat). This has the effect of recording plus cross coupling from one meter versus minus cross coupling from the other meter, or simultaneous accelerations in two different directions. The integration of the two gravity recordings cancels random noise and improves the quality of the gravity data. This is necessary in order to take advantage of new sophisticated computer applications in interpretation.
These data are normally tied to absolute gravity values at International Gravity Bases located at ports of call at about one month intervals. There is a need to increase the frequency of the base ties which increases the accuracy of the shipboard gravity data, in addition to using two gravity meters.
The customary procedure for tying the marine gravity survey to the International Gravity Base Network is to take a still gravity reading with the shipboard gravity meter at a port that has an established international gravity base value. The difference between the still gravity reading taken with the shipboard gravity meter on the boat and the international gravity value is applied to the shipboard gravity readings to place these values on the international gravity datum.
As a general rule the boat returns to this port, or another port that has an established international gravity base value, within 30 days. If the gravity tie is made to another base, the difference between the two international gravity bases is simply applied to the original base which makes it the same as making both ties to the original base. At that time a repeat still gravity reading is recorded with the shipboard gravity meter at the gravity base. The difference between the gravity value determined at the first tie at the gravity base and the second tie at the gravity base gives the amount of drift, or instrument reading change, and the diurnal change in the gravity field for the time interval between the two base gravity readings. The 30 day period for checking the amount of change in instrument drift and the diurnal change in the gravity field is insufficient for a detailed gravity survey.
Part of our invention to improve shipboard gravity data includes setting up a gravity base network at sea. Part of the new bases at sea are tied directly to the International Gravity Base Network at ports of call. Then a systematic grid of bases is developed at sea. The base values at sea are determined by taking still readings with the shipboard gravity at locations on a regular grid within the area to be recorded in a moving mode by the shipboard gravity meter. The network of gravity bases established at sea are all tied together and any differences above 0.1 milligal are resolved. Before the actual shipboard survey starts, continuous recordings are made down each of the lines of the base grid and these are adjusted to the base network at sea. An appropriate grid for the sea gravity bases is six miles which provides continuous gravity control that is very accurately tied. This now makes it possible to cross a gravity base line approximately every 30 minutes if the regular shipboard survey is conducted at ten miles per hour, or approximately every hour if the survey is conducted at five miles per hour. The gravity diurnal and instrument drift can be corrected more accurately by having the base gravity ties at 30 minutes to one hour versus one month.
The purpose of the Multifold Shipboard Gravity updated with still readings, or bases, is to acquire marine gravity data that is superior to existing marine gravity data.
The proposed procedures will increase the accuracy of the gravity data to about 0.25 milligals from an average of 0.50 milligals or more. Gravity will be recorded using proven methods to increase the accuracy. The procedures described will make the cost higher than a conventional gravity survey to achieve the accuracies sought; however, the high quality data will be worth more than conventional data.
Continuous base gravity readings will also be digitally recorded on land at a base station using a LaCoste & Romberg Geodetic Gravity Meter. This will give a portrayal of the diurnal gravity changes for the area at a stable fixed location. This will also be integrated with the shipboard gravity data when there are significant changes in the diurnal gravity data.
It is necessary to have a very stable boat that has adequate space in the lower part, or center of gravity, for two or more LaCoste & Romberg Air/Sea Gravity Meters. The navigation will be Differential Global Positioning Service using satellite positioning with land station updates. The horizontal accuracy of the positions is three to nine meters when there are sufficient satellites. Recording will be shut down a couple of hours each day when there are few useable satellites. The precision bathymetric system using sound velocity will give bathymetric data accurate to +/- one meter or less. The depths at the still readings will be measured exactly and integrated with other depths for an accuracy of one meter.
The meters will be tuned to read the same values and times. One meter will face the front of the boat and one will face the back of the boat for plus and minus cross coupling, or simultaneous accelerations in two different directions. The gravity from the meters will be integrated to cancel random noise. Our standard marine gravity survey is normally accurate to 0.5 milligals. The meters recording data simultaneously, or multifold gravity will produce shipboard gravity accurate to about 0.35 milligals. The combined shipboard gravity data tied to the grid of still gravity readings will be accurate to about 0.25 milligals.
A three minute bell shaped filter will be appropriate for the gravity data. This will be 0.5 mile width at ten knots and will define gravity anomalies from geologic structure deeper than about 660 feet deep. This filter would not remove anything deeper than 660 feet in water from 600 to over 6,000 feet.
These are the points we have highlighted to improve the accuracy of the gravity data:
1. Closely spaced gravity control PA0 2. Stable research vessel PA0 3. Fully equipped vessel with functioning auxiliary equipment in place PA0 4. Boat crew familiar with area PA0 5. Perfect space for two or more shipboard meters in stable location on vessel PA0 6. Ties to absolute gravity PA0 7. Ties to still gravity readings PA0 8 Still readings in tied loops for gravity meter drift control PA0 9. Base gravity meter to measure diurnal gravity on land PA0 10. Experienced gravity meter operators PA0 11. Recent vintage shipboard gravity meters PA0 12. Gravity meter maintenance PA0 13. Multiple meters PA0 14. Multifold gravity PA0 15. Meter arrangement for maximum random noise cancellation PA0 16. Improved navigation with differential updates PA0 17. Improved bathymetry with sound velocity calibrations PA0 18. Pause in production during sparse satellite control determinations from logs and cores for precise Bouguer correction PA0 20. Experience in acquisition, compilation and interpretation